VirtualBox requires a kernel module that is not available for OpenBSD, it also has a lot of other dependencies.. you will not be able to use it on OpenBSD.
QEMU (..currently outdated) with kqemu may work, although there have been issues with kqemu on MP kernels.. and possibly amd64.
Theo has also stated that the emulator has several bugs preventing OpenBSD from running properly inside it as well.
As for the new disk, it is very easy to use it the way you want.. prepare the disk using
fdisk(8)+
disklabel(8) and
newfs(8) as normal, copy over any existing data using
cp(1) or
dump(8)/
restore(8) and then alter your
/etc/fstab file accordingly.
After you've done that, just reboot.